Field of the Invention
The present invention concerns the use of a modified TrueFISP sequence, in order to capture MR data simultaneously from multiple slices of a subject. In this case, a TrueFISP sequence is understood to mean a sequence for operating a magnetic resonance installation that produces gradient moments that are balanced along all three spatial axes.
Description of the Prior Art
MR raw data capture (acquisition) using a TrueFISP sequence (True Fast Imaging with Steady State Precession) is characterized by a very high signal to noise ratio and high speed. In order to increase the speed further, parallel data capture is required, in which raw data from at least two slices are captured simultaneously.
The manner by which an MR data acquisition scanner is operated in order to execute such a data acquisition sequence (or any MR data acquisition sequence) is defined by an associated control protocol that specifies, for the scanner, the sequence execution.
In the case of parallel data capture, it is necessary to solve the problem of proportionally assigning the received MR signals to the various simultaneously excited slices. One method that assists this assignment or signal separation over the various slices is the so-called CAIPIRINHA method (“Controlled Aliasing in Parallel Imaging Results in Higher Acceleration for Multi-Slice Imaging”, Magn. Reson. Med. 2005; 53 (3), F. A. Breuer et al., pages 684-691).
A further known method that assists this assignment or signal separation over the various slices is described in US 2013/0271128 A1.
The means by which this CAIPIRINHA method can be modified such that it can also be used for TrueFISP sequences can be found in “CAIPIRINHA accelerated SSFP imaging”, Magn. Reson. Med. 2011; 65, D. Stäb et al., pages 157-164, for example.
In this modified CAIPIRINHA method, provision is first made for varying the phase of consecutive RF excitations of the same slice. Secondly, the phases of the simultaneously occurring RF excitations of the slices to be captured simultaneously are shifted relative to each other. When capturing the MR data from a slice S0 and a slice S1, the phase PS0 of the RF excitations of the slice S0 can satisfy e.g. the following equation (1), while the phase Psi of the RF excitations of the slice S1 can satisfy e.g. the following equation (2).PS0=−k*90°  (1)PS1=+k*90°  (2),where k designates the running index, i.e. the phase PS0 or PS1 changes from repetition time-to-repetition time by a phase increment of −90° or +90° respectively. The difference in the phase increments would therefore be 180° in this case.
FIG. 1 illustrates the frequency bands of the two slices S0 and S1 to be captured simultaneously. The operational signs within the frequency bands indicate whether the amplitude of the captured MR signals is positive or negative. The reference sign 31 designates the dark bands.
It can be seen from FIG. 1 that the modified CAIPIRINHA method shifts the band structure of the two slices S0, S1 by ¼ of the bandwidth in each case, the band structure of the slice S0 being shifted by +90° and the band structure of the slice S1 by −90° relative to the band center (0°). As result, the effective bandwidth 32 for both slices S0, S1 is reduced by approximately 50% in comparison with the bandwidth for only one of the slices.